A conventional current differential relay for a power transmission line is provided at both ends of a protected area on a power transmission line (hereinafter, “both ends of the power transmission line”). The current differential relays obtain electric current information from a current transformer used for obtaining a power-transmission-line current into the relay, mutually transmit the electric current information to the other end via a PCM transmission path, and compute a differential current between the currents flowing into both ends of the power transmission line to detect whether there is an internal fault in the power transmission line. When the internal fault is detected, the current differential relays operate and open a breaker provided at both ends of the power transmission line to cut off the power-transmission-line segment from the electric power system. In order to compute the differential current, the current differential relays provided at both ends of the power transmission line are controlled such that the sampling timings of analog-to-digital (AD) conversion of a current that is input to these current differential relays are synchronized with each other.
In conventional techniques, there have been proposed a method of synchronizing the above sampling timings by using a PCM transmission device that is specially configured to match the transmission delay times in a PCM transmission path between one end and the other end and a method of, in the case where the transmission delay times differ from each other between one end and the other end, synchronizing the above sampling timings by inputting a global-positioning-system (GPS) time signal and using the time signal that is input at the same time to the current differential relays provided at both ends of the power transmission line (for example, Patent Literature 1).
As a conventional technique, there has been proposed a method of synchronizing the above sampling timings by using a 1-pulse-per-second (1PPS) signal as the GPS time signal in the above latter technique to generate a 50-Hz or 60-Hz clock or a 600-Hz or 720-Hz clock from the 1PPS signal (for example, Patent Literature 2). The 1PPS signal is a highly accurate 1-pulse-per-second time signal generated on the basis of the GPS time signal. However, the GPS time signal is transmitted from the satellite and is therefore affected by the location of the satellite, weather, and other factors. Accordingly, the GPS time signal is not stable enough to be constantly received. Thus, in the conventional technique in Patent Literature 1, a current differential relay is configured to be able to continue its operation even when the GPS time signal is lost.
Meanwhile, a protective relay in which its functions are divided into two main functions is referred to as “process bus-compatible protective relay”. The process bus-compatible protective relay is constituted by a merging unit (MU: Merging Unit) and a relay computation unit (IED: Intelligent Electric Device). The merging unit is provided adjacent to the main unit body in a gas insulated substation (GIS) or the like, which is configured from a current transformer and other devices in a substation, and converts current and voltage input data to digital data to transmit the digital data to a communication circuit. The relay computation unit is provided at a location apart from the MU (for example, in a protection control room in the substation) and performs relay computation. The MU and the IED are communicated according to the protocol referred to as a process bus defined in the IEC 61850 communication standards.